To prevent bleeding in hæmophilics, intra-venous or subcutaneous injections of fresh blood serum, taken from the human subject, the sheep, the dog, or the horse, have proved useful. If fresh serum is not available, anti-diphtheritic or anti-tetanic serum or trade preparations, such as hemoplastin, may be employed. We have removed the appendix and amputated through the thigh in hæmophilic subjects without excessive loss of blood after a course of fresh sheep's serum given by the mouth over a period of several weeks. The chloride and lactate of calcium, and extract of thymus gland have been employed to increase the coagulability of the blood. The patient should drink large quantities of milk, which also increases the coagulability of the blood. Monro has observed remarkable results from the hypodermic injection of emetin hydrochloride in ½-grain doses. THROMBOSIS AND EMBOLISM The processes known as thrombosis and embolism are so intimately associated with the diseases of blood vessels that it is convenient to define these terms in the first instance. #Thrombosis.#--The term _thrombus_ is applied to a clot of blood formed in the interior of the heart or of a blood vessel, and the process by which such a clot forms is known as _thrombosis_. It would appear that slowing or stagnation of the blood-stream, and interference with the integrity of the lining membrane of the vessel wall, are the most important factors determining the formation of the clot. Alterations in the blood itself, such as occur, for example, in certain toxæmias, also favour coagulation. When the thrombus is formed slowly, it consists of white blood cells with a small proportion of fibrin, and, being deposited in successive layers, has a distinctly laminated appearance on section. It is known as a _white thrombus_ or laminated clot, and is often met with in the sac of an aneurysm (Fig. 72). When rapidly formed in a vessel in which the blood is almost stagnant--as, for example, in a pouched varicose vein--the blood coagulates _en masse_, and the clot consists of all the elements of the blood, constituting a _red thrombus_ (Fig. 66). Sometimes the thrombus is _mixed_--a red thrombus being deposited on a white one, it may be in alternate layers. When aseptic, a thrombus may become detached and be carried off in the blood-stream as an embolus; it may become organised; or it may degenerate and undergo calcification. Occasionally a small thrombus situated behind a valve in a varicose vein or in the terminal end of a dilated vein--for example in a pile--undergoes calcification, and is then spoken of as a _phlebolith_; it gives a shadow with the X-rays. When infected with pyogenic bacteria, the thrombus becomes converted into pus and a localised abscess forms; or portions of the thrombus may be carried as emboli in the circulation to distant parts, where they give rise to secondary foci of suppuration--pyæmic abscesses.